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AMI_1575 09-22-2008 09:58 AM

Exhaust Fan For Bathroom
I am wondering if someone could help me to understand the best way to install an exhaust fan in a main floor bathroom. Our main floor bath is right in the center of the home. I feel we should go straight up, and through the roof with it.(home is a rambler with an attic, though I don't believe you could stand up in the attic all the way) It is the shortest distance, seems it would help the fan work as effectively as it can. My husband is adamant that we go up (into the attic) and over, out the side of the house with the exhaust "duct work", which is approximately 20 feet. We are not able to go directly out the side of the house where the bathroom is because of the pitch of the roof.
I am not believing that an exhaust fan is capable of removing moisture, and successfully blowing it 20 feet sideways to get it outside. What would be the correct way to do this? Could anyone help me to understand? Thanks!

Square Eye 09-22-2008 03:54 PM

Build a few homes and you will run into this same problem sooner or later.

Running straight up and out may be the shortest distance but during cooler seasons, as the warm and steamy air moves up the exhaust duct, it will cool and the condensation will run back down the duct and could appear as a leak. If you go up into the attic and across, you have an easy opportunity to lay insulation directly on the duct and reduce (not eliminate) the condensation. But at 20' you will certainly still have the potential for a moisture problem.

Either way you go, Do seal the exhaust duct to the fan box and all joints in whatever type of ducting you use with a good quality foil tape. At least then, when/if the moisture problems surface, they will be manageable without destroying ceilings or worse.

AMI_1575 09-23-2008 07:41 AM

Thank you for your information, Square Eye. It is kind of sounding like this may not be a problem free project, whichever way we go.

Hube 09-23-2008 08:09 AM

Best to go staight up THRU the roof, and exhaust to the OUTSIDE. Insulate the ENTIRE piping from the fan to the roofline with a fibreglass insulation wrapping. This will avoid any chance of condensation.
Also make sure there is a "backdraft) flapper at the roof line exit.

AMI_1575 09-23-2008 09:42 AM

Thank you. When going straight thru roof, would you use blackjack to seal around the outside, or is there a better way to seal the hole?

Hube 09-23-2008 11:47 AM

Depends on what type of installation you do.
#1(best) is to extend the pipe approx 3 ' or so above the roof line (shingles) Make sure you install the proper FLASHING and weather SKIRT at the roof line(shingles)that will surround the protruding pipe. This flashing will extend under the high part of the shingles and the lower part of the flashing will lay on top of thel ower shingles, thus a weather tight installation. Put a weather cap on the very top of the pipe. The only caulking needed in this method is where the flashing weather skirt contacts the exhaust pipe.

#2 way is to purchase the "weather hood" type of exhaust termination fitting that simply sits on the shingles and the flashing is done the same as #1 method.
Personally, I like the #1 method best because it being 3' above the shingles there is no chances of blockage if in a snow belt climate.

woodchuck 09-24-2008 08:58 PM

Maybe it shouldn't have been done but our two bath vents are vented to the attic with no duct work. Thirty years and I haven't seen any problems.

mikemeier 09-24-2008 09:52 PM

If they vent to the attic there is usually moisture trouble. Probably the only reason woodchuck doesn't have any trouble is there is probably more than adequate venting and he doesn't live in an area that freezes during the winter. I would imagine that in MN you would end up with a lot of condensation during frosts.

Square Eye 09-25-2008 06:12 PM

I remember tearing a roof off down to the ceiling joists, the homeowner thought he had a leaking roof. What he had was a roof with no venting at all, unless you count the bathroom exhaust fan that had filled the attic space with enough moisture to rot the roof deck through from the inside out. The rafters were sagging and the insulation was still wet. Vent to the outside or not at all.

Still wasn't as bad as the dryer vent that was terminated under a house where the lady was doing laundry for 3 other families

inspectorD 09-26-2008 06:26 AM

Being in the business, we see alot of what goes wrong as Sqaure Eye shared. I have even seen 2 foot icicles hanging off roofing nails in the joke...from an exaust fan in the bathroom not vented to the exterior. The lady called the roofer, had the roof changed with ventilation. I went back, the fan was still not vented, and now the icicles where only 6 inches. :D Hmmm...vent to the outside.:D

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